Nader Al-Bastaki

Modeling an industrial reverse osmosis unit

A simplified model was used to predict the performance of hollow fiber RO membranes. The model is based on the solution-diffusion membrane mass transport model and takes into account the effect of the pressure drop in the fiber bore and in the fiber bundle, the variation of the bulk solution concentration on the shell side of the fiber bundle and the concentration polarization. The results of the model showed reasonable agreements with the experimental data for B9 membranes and the data from an industrial RO plant based on B10 membranes.

Use of fluid instabilities to enhance membrane performance: a review☆

Abstract This paper presents a review of some of the key methods of generating flow instabilities that have been implemented in membrane separation processes by various workers during the past few years (mainly the 1990s). The types and effects of incorporation these techniques are discussed based on the experimental and theoretical investigations in different applications, particularly, production of potable water from natural fresh water sources, water desalination, and purification of biological and organic solutions and suspensions. The main types of instabilities that are discussed are Dean vortices in helical hollow-fiber membranes, air sparging, novel backflushing techniques and cyclic feed operation. There is a substantial amount of data in the reviewed literature in support of improved performance and fouling prevention in the presence of flow instabilities.

Predicting the performance of RO membranes

Theoretical and experimental recoveries are compared for spiral-wound and hollow-fiber membranes. The effect of ignoring concentration polarization and pressure drops is studied. Ignoring concentration polarization and pressure drops results in significant overestimation of the recovery. With spiral-wound membranes pressure drops were found to be less significant. In addition, a solution method based on integration was used for hollow-fiber membranes. Each of the water and salt fluxes is defined as an implicit function of two dimensions, namely length and radius, and this function is integrated over the whole of the membrane. The integration method resulted in recovery predictions closest to the average of the experimental data. The agreement between predicted and experimental salt rejection was less pronounced, the theoretical values being higher than the experimental.

Performance decline in brackish water Film Tec spiral wound RO membranes

In this paper, long-term operational data from a medium scale RO (reverse osmosis) plant based on brackish water FilmTec spiral wound RO membranes are presented and analysed. It was found that over a period of 500 days of continuous operation, the water permeability coefficient for the overall plant decreased by about 25% and the salt rejection dropped by 1.9%. Simple formulae representing the long-term performance of the plant were developed. These expressions were found to fit the data with very good accuracy.

Long-term performance of an industrial water desalination plant

The long-term performance of a medium-scale industrial spiral wound reverse osmosis (RO) water desalination plant was studied. Operational data were analysed for a period of 1500 days as a basis for evaluating the performance variation with time. A theoretical model based on the solution-diffusion mass-transfer theory and concentration polarization was employed to extract the water and salt permeability coefficients. The results indicate that after over 4 years of continuous operation, the water permeability coefficient declined by about 39% and the average salt permeability coefficient increased by about 60%. Periodic membrane cleaning schemes were effective in partially restoring the water and salt permeabilities.

Permeate recycle to improve the performance of a spiral-wound RO plant

This paper presents the results obtained from a simulation study on the effect of permeate recycle on the performance of a spiral-wound RO plant. The model used was developed for an actual small scale plant consisting of three stages. The first stage consists of two parallel vessels whereas the second and third stages contain a single vessel each. Each vessel houses three 8-inch spiral-wound brackish water FilmTec membranes. Part of the product water is recycled and mixed with the fresh feed. The model incorporates the solution diffusion mass transport theory, concentration polarization, along with the characteristics of the spacer for the estimation of the mass transfer coefficient and pressure drop. The use of these concepts together with the recycling of a portion of the permeate resulted in a model composed of a highly coupled system of nonlinear equations. The results of the simulation showed that recycling part of the permeate and mixing it with the feed was beneficial in terms of reducing the concentration polarization and improving the quality of the final product. However, as expected, this resulted in reducing the production rate. By using a recycle ratio of 25%, the concentration of the product was reduced by 15% compared to the case of no recycle, at the expense of 22% reduction in the production rate.

Treatment of synthetic industrial wastewater with UVTiO2 and RO using benzene as a model hydrocarbon

Abstract Contamination of water resources with hazardous chemicals is one of the main concerns of all private and public sectors involved in the water production for drinking, irrigation and domestic use. The conventional disinfection process using chlorination adds to the complexity of the problem as some of these chemicals are precursors for disinfection by products such as trihalomethanes. This paper describes the work done to study the efficiency of two types of processes in removing a model hydrocarbon, namely, benzene, which was added to water at a concentration of 100 ppm. The two processes investigated were RO and UV/TiO2 The RO process was based on a FilmTec SW30 spiral wound membrane, which was operated at pressures of 20,40 and 60 bar. The UV/TiO2 employed was PhotoCat, manufactured by Purifics, Canada. The heart of the PhotoCat unit was 32 low pressure, low intensity mercury vapor lamps connected in series. Gas chromatography was used to analyze the results. The results showed that the UV/TiO2 resulted in a removal of 99.28%. However, the RO performance was less impressive with a maximum separation of 82.3%. At 40 bar, a negative permeability was observed, as the concentration of benzene in the permeate was higher than the feed. This latter result shows that care has to be taken with hydrocarbons polluting the feed water to RO units, since in certain cases the product water will more polluted than the feed.

Flux enhancement of RO desalination processes

Abstract The simple model proposed by Kennedy et al. was modified so as to improve its ability to predict the periodic performance operation of RO systems. The modified model was validated using data from the literature as well as data obtained from experiments in this work. The predicted permeate rates compared well with the experimental values for the considered forcing functions: sine and asymmetric square waves. Periodic and steady-state experimental runs were performed on a spiral-wound RO membrane system at three average pressures: 20, 25 and 30 bars. For the case of periodic operation, the operating pressure was varied according to an asymmetric square wave having a period of 5 min. The duration of each run was 30 min. The effect of the time split, γ, of the forcing wave on the permeation rate as well the salt rejection was investigated. The results showed that periodic operation leads to improvements in the permeation rate. For the tested pressures, the overall maximum water production rates occurred at a time split approximately equal to 0.65. It was also found that cyclic operation may lead to small reductions in the salt passage.

Assessment of thermal effects on the reverse osmosis of salt/water solutions by using a spiral wound polyamide membrane

Abstract The effect of varying the temperature of the feed water for reverse osmosis was studied for two aqueous solutions, one with a relatively high concentration of NaCl (5 g/1) and the second with a low salt concentration (0.8 g/1). The feed temperature was varied between 12–30° C and pressures of 20, 40 and 60 bars were employed. It was found that increasing the temperature resulted in an increase in both the salt passage and the permeate flow rate with increasing temperature. A sieve type mechanism can be used to qualitatively explain these results in terms of an increase of the pore size with temperature, thus allowing for more salt passage and permeate flow rate.

Effect of aging on the performance of RO hollow fiber membranes in a section of an RO plant

Abstract This study investigates the effect of the aging factor on operating parameters of RO permeators based on hollow fiber membranes that have been in operation for a period of 35,000 h. The ratios of salt passages, feed pressures, and permeate flow rates over their corresponding initial values were studied for 35,000 h. It was found that the salt passage and pressure increase whereas the permeate flow rate decreases with operating time, particularly during the initial period of operation, while the rates of change of the above parameters were smaller during subsequent periods. Water and salt permeabilities were calculated and plotted against time. It was found that water permeabilities decreased and salt permeabilities increased with time. The variation in the performance characteristics of the membranes is due to the physical changes in the hollow fiber material which resulted from continuous applied pressure, the influences of the chemicals and ions that are present in the water, and regular chemical treatments.